Seismic waves, Biology

Earthquake creates seismic waves that travel through and around the surface of the earth. Seismic waves are the waves of energy caused by sudden breaking of rocks during an earthquake. These waves move in all directions with different velocities through the earth's layers, reflecting and refracting at each interface. These waves are mainly of two types, body waves and surface waves. Body waves can travel through the earth's inner layer while latter are restricted to near the earth's surface.

Body waves

These waves move through the earth's interior. Body waves have greater frequency than surface waves. These waves consist of two types namely longitudinal P waves and transverse S waves. P waves or primary waves are compressional waves which means that the material particles undergo extensional and compressional strain along the direction of energy transmission i.e. particles moves in the same direction that the waves is moving. P waves, like sound waves are able to move through both liquid and solid rocks. The P waves travel much faster than S waves and surface waves.

S waves or secondary waves are the waves directly following the P waves. These are also known as shear waves. They are most damaging waves which move near the earth's surface. As they move, S waves shear or cut the rock they travel through sideways at right angles to the direction of the wave and structures to vibrate from side to side. S waves can only move through solid rock not through liquid medium because liquid can be compressed but can't be sheared. S waves produce vertical and horizontal motion in the surface of earth, hence S wave are more dangerous than P waves.

Surface waves

Waves that travel only through earth's crust. Surface waves consist of two waves as love waves and Rayleigh waves. (fig.5.5). love waves move like S waves but with no vertical component. Rayleigh waves move both horizontally and vertically. These waves make a particle move in an elliptic path in the vertical plane with horizontal motion along the direction of travel of energy. These waves are usually recorded at a greater distance from the epicenter.